1. Field of the Invention
The present invention relates to a vehicle charging and generating system, and more specifically, to an indication device for such a charging and generating system adapted to indicate the power generating capacity of an AC generator for preventing the overdischarge of a storage battery which would occur even though the AC generator is normally operating.
2. Description of the Related Art
FIG. 2 shows a typical example of an electrical circuit of a vehicle charging and generating system. The system illustrated includes an AC generator 1 which has an armature coil 101 and a field coil 102 and which is adapted to be driven by an unillustrated engine installed on a vehicle.
A commutator 2 for performing the full-wave commutation of the AC output of the AC generator 1 has a main output terminal 201 for outputting main power, an auxiliary terminal 202 for energizing the field coil 102 and imposing a commutated output voltage of the AC generator 1 onto a voltage regulator 3 which will be described in detail later, and a ground terminal 203 connected to ground.
The voltage regulator 3 is to regulate the output voltage of the AC generator 1 at a prescribed level, and comprises a pair of voltage-dividing resistors 301, 302 connected in series with each other between the auxiliary output terminal 202 and the ground for dividing the output voltage from the auxiliary output terminal 202 of the commutator 2. A zener diode 303 is connected between the junction of the voltage-dividing resistors 301, 302 and the base of a control transistor 304 for detecting, together with the voltage-dividing resistors, the output voltage of the AC generator 1. The control transistor 304 is turned on and off depending upon conduction and non-conduction of the zener diode 303, and has its emitter connected to ground, and its collector connected to the base of a power transistor 305 and to the auxiliary output terminal 202 of the commutator 2 through a base resistor 306 which is the base resistance of the power transistor 305.
The power transistor 305 has its emitter connected to ground and its collector connected to one end of the field coil 102. Specifically, the power transistor 305 is series connected with the field coil 102 and is turned on and off by means of the control transistor 304 so as to control the field current of the field coil 102.
A diode 307 is connected in parallel to the field coil 102 for absorbing discontinuous surges developing in the field coil 102.
The system further includes a battery 4, a key switch 5 and an indicator lamp 6 all of which are connected in series with each other between the auxiliary output terminal 202 and ground. Also, electric loads 7 and a load switch 8 are connected in series with each other but in parallel with the battery 4 the positive terminal of which is connected to the main output terminal 201 of the commutator 2.
FIG. 3 shows a maximum output current characteristic of the AC generator 1, i.e., the relationship between the output current and the rotational speed (rpm) of the generator which has a saturated maximum output current of 100 A. Also, shown in this figure is a total amount of electric load on the vehicle side with respect to the output current of the AC generator 1. The total electric load is generally set at 75% of the saturated maximum output current of the generator 1, i.e., 75 A or therearound.
Now, the operation of the above-described conventional vehicle charging and generating system will be described below. When the key switch 5 is closed for starting the engine, an initial energization current begins to flow from the battery 4 to the field coil 102 via the key switch 5 and the indication lamp 6 so that the AC generator 1 becomes ready to generate power and the indication lamp 6 is illuminated, indicating a state of non-power generation.
Subsequently, when the engine is started, the AC generator 1 begins to generate electrical power and the voltage at the auxiliary output terminal 202 of the commutator 2 rises. Simultaneous with this, the electric potential difference across the opposite ends of the indication lamp 6 decreases toward a state of equilibrium and the indication lamp 6 is put out, indicating the starting of normal power generation of the AC generator 1.
The voltage regulator 3 detects the output voltage of the auxiliary output terminal 202 of the commutator 2 by the voltage-dividing resistors 301, 302 and the zener diode 303. When the output voltage of the auxiliary output terminal 202 exceeds a prescribed level which is set by the voltage-dividing resistors 301, 302 and the zener diode 303, the zener diode 303 is made conductive so that the control transistor 304 is driven into a conductive state.
On the other hand, when the output voltage of the auxiliary output terminal 202 falls below the prescribed level, the zener diode 303 is made non-conductive so that the control transistor 304 is made into a non-conductive state.
As described above, by the conduction and non-conduction of the control transistor 304, the power transistor 305 is turned on and off to control the field current flowing through the field coil 102 whereby the output voltage of the AC generator 1 is adjusted to a predetermined level.
During operation, when a vehicle load 7 such as lights, motors and the like is connected or turned on by closing a load switch 8, the output voltage of the AC generator 1 drops. In order to compensate for such a drop of the output voltage, the conduction rate (i.e., the capacity for allowing a current to flow through) of the power transistor 305 increases so as to increase the field current.
In other words, the conduction rate of the power transistor 305 changes in such a manner as to balance with the amount of electric load 7 which has been turned on, thereby adjusting the output of the AC generator 1 in an increasing or decreasing sense. Accordingly, when the conduction rate of the power transistor 305 is 100%, the output of the AC generator 1 reaches its maximum which varies characteristically with respect to the rotational speed of the AC generator 1 in the manner as shown in FIG. 3.
In this connection, the total or full amount of the electric load 7 on the vehicle side is generally set at about 75 of the saturated maximum output of the AC generator 1. Accordingly, in the case illustrated in FIG. 3, the maximum output of the AC generator 1 is in balance with the total or full electric load at a rotational speed of 2,000 rpm, and hence the output of the AC generator 1 is adequate to the total electrical load at a rotational speed equal to or more than 2,000 rpm, but inadequate at a rotational speed therebelow.
In this situation, there is no problem if the duration of a low-speed operating mode of the engine is equal to or shorter than that of a high-speed operating mode, but for example, in cases where the low-speed operating mode continues for an extended period of time under heavy electric load, it is a matter of course that the output of the AC generator 1 will not be sufficient for such a load, and this must be compensated for by a discharge of the battery 4.
In such a case, however, despite the battery 4 being discharged, the operator of the vehicle is not made aware of this and continues to drive the vehicle because no provision is made for indicating such a situation. Finally, the battery 4 is overdischarged and exhausted, making the vehicle inoperative.
The above-described vehicle charging and generating system, which is not provided with any indicator means for indicating the relation between the amount of power generated by the generator 1 and the amount of electric load connected or turned on, has the following problem; in cases where the vehicle is operated in a low-speed operating mode with a heavy electric load for an extended period of time, the operator tends to continuously drive the vehicle without noticing that the battery 4 is being discharged. As a result, the battery 4 is finally overdischarged and exhausted so that the vehicle cannot travel any more.